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Ohms Law

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  • 1. Ohm’s Law Mitsuko J. Osugi Physics 409D Winter 2004 UBC Physics Outreach
  • 2. Ohm’s Law
    • Current through an ideal conductor is proportional to the applied voltage
      • Conductor is also known as a resistor
      • An ideal conductor is a material whose resistance does not change with temperature
    • For an ohmic device,
      • V = Voltage (Volts = V)
      • I = Current (Amperes = A)
      • R = Resistance (Ohms = Ω )
  • 3. Current and Voltage Defined
    • Conventional Current : (the current in electrical circuits)
    • Flow of current from positive terminal to the negative terminal.
    • - has units of Amperes (A) and is measured using ammeters .
    • Voltage :
    • Energy required to move a charge from one point to another.
    • - has units of Volts (V) and is measured using voltmeters .
    Think of voltage as what pushes the electrons along in the circuit, and current as a group of electrons that are constantly trying to reach a state of equilibrium .
  • 4. Ohmic Resistors
    • Metals obey Ohm’s Law linearly so long as their temperature is held constant
      • Their resistance values do not fluctuate with temperature
        • i.e. the resistance for each resistor is a constant
    • Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.
  • 5. Voltage and Current Relationship for Linear Resistors Voltage and current are linear when resistance is held constant.
  • 6. Ohm’s Law continued
  • 7. Ohm’s Law continued
    • The total resistance of a circuit is dependant on the number of resistors in the circuit and their configuration
      • Series Circuit
      • Parallel Circuit
  • 8. Kirchhoff’s Current Law
    • Current into junction = Current leaving junction
    The amount of current that enters a junction is equivalent to the amount of current that leaves the junction
  • 9. Kirchhoff’s Voltage Law
    • Net Voltage for a circuit = 0
    Sum of all voltage rises and voltage drops in a circuit (a closed loop) equals zero
  • 10. Series Circuit
    • Current is constant
    • Why?
      • Only one path for the current to take
  • 11. Series Equivalent Circuit
  • 12. Parallel Circuit
    • Voltage is constant
    • Why ?
      • There are 3 closed loops in the circuit
  • 13. Parallel Equivalent Circuits
  • 14.
    • We’ve now looked at how basic electrical circuits work with resistors that obey Ohm’s Law linearly.
    • We understand quantitatively how these resistors work using the relationship V=IR, but lets see qualitatively using light bulbs.
  • 15. The Light Bulb and its Components
    • Has two metal contacts at the base which connect to the ends of an electrical circuit
    • The metal contacts are attached to two stiff wires, which are attached to a thin metal filament.
    • The filament is in the middle of the bulb, held up by a glass mount.
    • The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon.
  • 16. Light bulbs and Power
    • Power dissipated by a bulb relates to the brightness of the bulb.
    • The higher the power, the brighter the bulb.
    • Power is measured in Watts [W]
    • For example, think of the bulbs you use at home. The 100W bulbs are brighter than the 50W bulbs.
  • 17. Bulbs in series experiment
    • One bulb connected to the batteries. Add another bulb to the circuit in series .
    • Q: When the second bulb is added, will the bulbs become brighter, dimmer, or not change?
    • We can use Ohm’s Law to approximate what will happen in the circuit in theory:
  • 18. Bulbs in series experiment continued…
  • 19. Bulbs in parallel experiment
    • One bulb connected to the batteries. Add a second bulb to the circuit in parallel .
    • Q: What happens when the second bulb is added?
    •  We can use Ohm’s Law to approximate what will happen in the circuit:
  • 20. Bulbs in parallel experiment continued…
  • 21. Light bulbs are not linear
    • The resistance of light bulbs increases with temperature
    The filaments of light bulbs are made of Tungsten, which is a very good conductor. It heats up easily.
  • 22. As light bulbs warm up, their resistance increases. If the current through them remains constant:
    • They glow slightly dimmer when first plugged in.
      • Why?
      • The bulbs are cooler when first plugged in so their resistance is lower. As they heat up their resistance increases but I remains constant  P increases
    • Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.
  • 23. Voltage versus Current for Constant Resistance The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.
  • 24. “ Memory Bulbs” Experiment
    • Touch each bulb in succession with the wire, each time completing the series circuit
    • Q: What is going to happen?
    • Pay close attention to what happens to each of the bulbs as I close each circuit.
  • 25. “ Memory Bulbs” Continued…
    • Filaments stay hot after having been turned off
    • In series, current through each resistor is constant
      • smallest resistor (coolest bulb) has least power dissipation, therefore it is the dimmest bulb
    How did THAT happen?? Temperature of bulbs increases  resistance increases  power dissipation (brightness) of bulbs increases
  • 26. Conclusion
    • Ohmic resistors obey Ohm’s Law linearly
    • Resistance is affected by temperature. The resistance of a conductor increases as its temperature increases.
    • Light bulbs do not obey Ohm’s Law linearly
      • As their temperature increases, the power dissipated by the bulb increases
        • i.e. They are brighter when they are hotter
  • 27. You’re turn to do some experiments!
    • Now you get to try some experiments of your own, but first, a quick tutorial on the equipment you will be using
  • 28. The equipment you’ll be using:
    • - Voltmeter
    • - Breadboard
    • - Resistors
    • - 9V battery
    • Let’s do a quick review…
  • 29. How to use a voltmeter:
    • Voltmeter:
    • - connect either end of the meter to each side of the resistor
    • If you are reading a negative value, you have the probes switched.
    • There should be no continuity beeping . If you hear beeping, STOP what you are doing and ask someone for help!
  • 30. Voltmeter
  • 31. Measuring Voltage Voltage: Probes connect to either side of the resistor
  • 32. Breadboards
    • You encountered breadboards early in the year. Let’s review them:
    The breadboard How the holes on the top of the board are connected:
  • 33. Series Resistors are connected such that the current can only take one path
  • 34. Parallel Resistors are connected such that the current can take multiple paths
  • 35. Real data
    • In reality, the data we get is not the same as what we get in theory.
    • Why?
    • Because when we calculate numbers in theory, we are dealing with an ideal system. In reality there are sources of error in every aspect, which make our numbers imperfect.
  • 36. Now go have fun!

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